NEW YORK, June 10 (Reuters) - Former Goldman Sachs Group Inc board member Rajat Gupta, on trial on insider trading charges, has decided not to testify in his own defense, two days after his lawyer said it was "highly likely" he would take the stand.

"After substantial reflection and consideration, we have determined that Mr Gupta will not be a witness on his own behalf in the defense case," Gupta's main lawyer, Gary Naftalis, wrote in a letter to U.S. District Judge Jed Rakoff on Sunday.

Gupta, 63, is fighting charges that he illegally divulged financial information to his one-time friend and business associate, Galleon Group founder Raj Rajaratnam, between March 2007 and January 2009 while serving on the boards of Goldman and Procter & Gamble Co.

The trial in U.S. District Court in New York began on May 21 and prosecutors rested their case o n Friday. As the defense began its leg of the case, Naftalis told the judge after the jury had gone home for the weekend that "it is highly likely my client will testify."

The trial resumes on Monday and the jury is expected to hear closing arguments o n Wednesday.

Rajaratnam, who was convicted a year ago of insider trading and is serving an 11-year prison term, opted not to testify in his own defense. If convicted on charges of securities fraud and conspiracy, Gupta faces a possible maximum 25 years in prison.

The case is USA v. Gupta, U.S. District Court for the Southern District of New York, No. 11-907.